Dielectric ceramic composition

ABSTRACT

A dielectric ceramic composition consists essentially of Pb(Ni 1/3  Nb 2/3 )O 3 , PbTiO 3 , Pb(Zn 1/2  W 1/2 )O 3  and Pb(Cu 1/3  Nb 2/3 )O 3 , the mole percentages of said four components being as follows: 
     Pb(Ni 1/3  Nb 2/3 )O 3  --50.0 to 75.0 mole % 
     PbTiO 3  --20.0 to 35.0 mole % 
     Pb(Zn 1/2  W 1/2 )O 3  --0.5 to 15.0 mole % 
     Pb(Cu 1/3  Nb 2/3 )O 3  --1.0 to 10.0 mole % 
     The dielectric ceramic composition may further contain, as an additive, manganese in an amount of not more than 0.5 mole % in terms of MnO 2 . The composition possesses a high dielectric constant of not less than 8000 and a low sintering temperature of 900° to 1000° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a dielectric ceramic composition and, moreparticularly, an improved dielectric ceramic composition with a highdielectric constant of not less than 8000 and a low sinteringtemperature of 900° to 1000° C.

2. Description of the Prior Art

Recently, electronic appliances for industrial and consumer uses havebeen required to be light-weight and compact and to have high qualityand high operation frequencies. For this reason, there is an increasingdemand for monolithic ceramic capacitors because of their largecapacitance-to-volume ratio, high mass producibility and excellentelectrical and physical characteristics including high frequencycharacteristics and heat resistance.

The monolithic ceramic capacitors are generally built up of anarrangement of alternating two layers of a dielectric layer, composed ofa dielectric ceramic composition of a barium titanate system, and aninternal electrode. The dielectric ceramic composition of the prior arthas a high sintering temperature of 1300° to 1400° C., so that it hasbeen required to use a noble metal such as Au, Pt and Pd as a materialfor internal electrodes. However, the noble metals are expensive to useand occupies the greater part of the manufacturing cost of themonolithic ceramic capacitors. Thus, the use of the noble metal sets alimit to lower the manufacturing cost of the monolithic ceramiccapacitors.

To solve this problem, it has been proposed to incorporate a glasscomponent comprising oxides of boron, bismuth and lead into the abovemodified barium titanate composition. The addition of the glasscomponent lowers the sintering temperature to 1100° to 1150° C., thusmaking it possible to use a relatively inexpensive silver-paradium alloyas a material for internal electrodes. However, the incorporation of theglass component causes lowering of the dielectric constant, and thus themonolithic ceramic capacitors increase in size for a given capacitance.This results in increase in an amount of the electrode material to beused, thus making it difficult to reduce the manufacturing cost of themonolithic ceramic capacitors.

It is an object of the present invention to provide a dielectric ceramiccomposition with a high dielectric constant of not less than 8000 and alow sintering temperature of 900° to 1000° C.

Another object of the present invention is to provide a dielectricceramic composition capable of being used as a dielectric material formonolithic ceramic capacitors comprising internal electrodes of arelatively inexpensive silver-paradium alloy.

SUMMARY OF THE INVENTION

According to the present invention there is provided a dielectricceramic composition consisting essentially of Pb(Ni_(1/3) Nb_(2/3))O₃,PbTiO₃, Pb(Zn_(1/2) W_(1/2))O₃ and Pb(Cu_(1/3) Nb_(2/3))O₃, the molepercentages of said four components being as follows:

Pb(Ni_(1/3) Nb_(2/3))O₃ --50.0 to 75.0 mole %

PbTiO₃ '20.0 to 35.0 mole %

Pb(Zn_(1/2) W_(1/2))O₃ --0.5 to 15.0 mole %

Pb(Cu_(1/3) Nb_(2/3))O₃ --1.0 to 10.0 mole %

The dielectric ceramic composition of the above four component systemmay further contain, as an additive, manganese in an amount of not morethan 0.5 mole % in terms of MnO₂ with respect to one mole of its basiccomposition consisting essentially of 50.0 to 75.0 mole % of Pb(Ni_(1/3)Nb_(2/3))O₃, 20.0 to 35.0 mole % of PbTiO₃, 0.5 to 15.0 mole % ofPb(Zn_(1/2) W_(1/2))O₃, and 1.0 to 0.0 mole % of Pb(Cu_(1/3)Nb_(2/3))O₃.

These and other objects, features and advantages of the presentinvention will be more apparent from the following description withreference to the preferred embodiments thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The dielectric ceramic composition of the present invention wereproduced in the following manner.

As starting raw materials there were used highly purified oxides(purity: not less than 99.9%) PbO, NiO, Nb₂ O₅, TiO₂, ZnO, WO₃, CuO andMnO₂. These raw materials were weighed to prepare a mixture for theproduction of the final product having a composition with thecompositional proportion shown in Table 1. Each of the resultant mixture(100 g) was wet milled in a polyethylene mill with agate balls for 10hours. The resultant slurry was dried, placed in an alumina saggar,calcined at 650° to 800° C. for 2 hours, and then crushed to preparepowder of the primary reactant (calcined powder). The calcined powderwas wet milled together with 3% by weight of polyvinyl alcoholincorporated therein as a binder, dried and then passed through a 50mesh sieve screen. The thus obtained granulated powder was pressed intodiscs having a diameter of 12 mm and a thickness of 1.2 mm under apressure of 2000 kg/cm² with an oil press. The discs were placed in azirconia saggar and then fired at 900° to 1100° C. for 2 hours with anelectric furnace in a lead-containing atmosphere.

Each of the resultant ceramic disc was provided on its both sides withsilver electrodes by applying silver paste containing borosilicate glassfrit and then baking the same at 750° C. for 10 minutes to preparespecimens for measurements of electrical properties.

The measurements were made on capacitance (C), dielectric loss (tan δ)and the insulation resistance (IR). The capacitance (C) and dielectricloss (tan δ) were measured at 1 KHz and 1 Vrms with an LCR meter, Model4274 made by YOKOGAWA HEWLETT PACKARD CO. The insulation resistance wasmeasured after applying a DC voltage of 500 V for 2 minutes with amegohom meter, Model TR 8601 made by TAKEDA RIKEN KOGYO CO. A thicknessof the ceramic disc and a diameter of the opposed electrodes weremeasured to determine the dielectric constant (ε) and the specificresistance (ρ). Results are shown in Table 2.

In Tables 1 and 2, specimens with an asterisk (*) are those having acomposition beyond the scope of the present invention, while otherspecimens are those included in the scope of the present invention.

                                      TABLE 1                                     __________________________________________________________________________    Specimen                                                                           Basic Composition (mol %)   Additive                                     No.  Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3                                                      PbTiO.sub.3                                                                        Pb(Zn.sub.1/2 W.sub.1/2)O.sub.3                                                      Pb(Cu.sub.1/3 Nb.sub.2/3)O.sub.3                                                      MnO.sub.2 (mol %)                            __________________________________________________________________________     1*  74      24   0      2.0     0                                            2    73.5    24   0.5    2.0     0                                            3    65      26   7.0    2.0     0                                            4    55      28   15.0   2.0     0                                             5*  50      28   20.0   2.0     0                                             6*  58      35   7.0    0       0                                            7    61      28   10     1.0     0                                            8    59      26   10     5.0     0                                            9    56      24   10     10      0                                            10*  53      24   10     13      0                                            11*  75      15   5.0    5.0     0                                            12   70      23   5.0    2.0     0                                            13*  54      39   5.0    2.0     0                                            14*  45      35   10     10      0                                            15*  78      20   1.0    1.0     0                                            16   64.9    26   7.0    2.0     0.1                                          17   64.7    26   7.0    2.0     0.3                                          18   64.5    26   7.0    2.0     0.5                                          19*  64      26   7.0    2.0     1.0                                          20   60.7    28   10     1.0     0.3                                          21   60.5    28   10     1.0     0.5                                          22*  60.0    28   10     1.0     1.0                                          __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        Specimen                                                                             Sintering          tanδ                                                                         ρ(Ω-cm)                              No.    Temp. (°C.)                                                                       ε                                                                             (%)  At 25° C.                                                                      At 85° C.                       ______________________________________                                         1*    1050       6,400   0.12 3.16 × 10.sup.13                                                                2.51 × 10.sup.12                  2     1000       8,900   0.37 1.58 × 10.sup.12                                                                3.29 × 10.sup.11                  3     950        14,300  0.74 2.39 × 10.sup.12                                                                5.01 × 10.sup.11                  4     900        10,800  1.00 6.31 × 10.sup.11                                                                7.88 × 10.sup.10                  5*    900        6,100   1.27 1.40 × 10.sup.9                                                                 7.92 × 10.sup.7                   6*    1060       19,000  1.25 3.73 × 10.sup.10                                                                6.42 × 10.sup.10                  7     950        15,200  0.80 6.25 × 10.sup.12                                                                4.99 × 10.sup.11                  8     900        12,300  0.67 1.15 × 10.sup.12                                                                1.19 × 10.sup.11                  9     900        8,900   0.40 1.62 × 10.sup.11                                                                1.01 × 10.sup.10                  10*   900        5,600   0.23 6.18 × 10.sup.9                                                                 5.12 × 10.sup.7                   11*   950        5,100   0.10 1.15 × 10.sup.12                                                                6.39 × 10.sup.10                 12     950        10,500  0.65 2.00 × 10.sup.12                                                                2.63 × 10.sup.11                  13*   950        6,900   4.20 2.71 × 10.sup.11                                                                1.13 × 10.sup.10                  14*   900        5,800   0.37 7.94 × 10.sup.10                                                                1.20 × 10.sup.9                   15*   1100       3,400   0.10 3.08 × 10.sup.13                                                                7.97 × 10.sup.12                 16     950        14,000  0.56 1.07 × 10.sup.13                                                                2.38 × 10.sup.12                 17     950        13,700  0.47 1.43 × 10.sup.13                                                                5.07 × 10.sup.12                 18     950        10,400  0.31 1.24 × 10.sup.13                                                                2.45 × 10.sup.12                  19*   950        6,100   1.25 3.21 × 10.sup.10                                                                1.71 × 10.sup.8                  20     950        14,200  0.66 3.98 × 10.sup.13                                                                1.60 × 10.sup.12                 21     950        10,700  0.70 3.33 × 10.sup.13                                                                1.30 × 10.sup.13                  22*   950        7,000   2.80 1.66 × 10.sup.10                                                                1.08 × 10.sup.9                  ______________________________________                                    

From the results shown in Table 2, it is apparent that the dielectricceramic composition according to the present invention has a highdielectric constant (ε) of not less than 8000 and a low sinteringtemperature ranging from 900° to 1000° C.

In the present invention, the mole percentages of the four components,Pb(Ni_(1/3) Nb₁₇₀ )O₃, PbTiO₃, Pb(Zn_(1/2) W_(1/2))O₃ and Pb(Cu_(1/3)Nb_(2/3))O₃ are limited to the ranges as defined above for the followingreasons.

Firstly, if the content of Pb(Ni_(1/3) Nb_(2/3))O₃ is less than 50 mole%, the dielectric constant becomes low and less than 8000 and theresultant dielectric ceramic compositions can not be put into practicaluse since they exhibit electrical characteristics equal to or less thanthose of the barium titanate compositions of the prior art. If thecontent of the content of Pb(Ni_(1/3) Nb_(2/3))O₃ is greater than 75.0mole %, the sintering temperature becomes higher than 1000° C. and thedielectric constant becomes extremely low.

Secondary, if the content of PbTiO₃ is less than 20.0 mole % or greaterthan 35.0 mole %, the dielectric constant lowers to less than 8000.

Thirdly, if the content of Pb(Zn_(1/2) W_(1/2))O₃ is less than 0.5 mole%, the sintering temperature become higher than 1000° C. If such acomposition is sintered at a temperature of less than 1000° C., theproduct exhibits low density and low dielectric constant less than 8000.If the content of Pb(Zn_(1/2) W_(1/2))O₃ is greater than 15 mole %, thedielectric constant becomes less than 8000 and the specific resistanceis low even though the sintering temperature becomes less than 1000° C.

If the content of Pb(Cu_(1/3) Nb_(2/3))O₃ is less than 1.0 mole &, thedielectric constant is high, but the sintering temperature becomeshigher than 1000° C. If the content of Pb(Cu_(1/3) Nb_(2/3))O₃ isgreater than 10.0 mole %, the composition exhibits good sinteringproperties, but its dielectric constant becomes less than 8000 and thespecific resistance lowers.

The added amount of manganese is limited to the above range for thefollowing reasons. If the added amount of Mn is greater than 0.5 mole %in terms of MnO₂ with respect to one mole of the basic composition ofthe four component system, the dielectric constant becomes less than8000, and the specific resistance lowers to less than 10¹¹ Ω-cm.

The dielectric ceramic composition of the present invention hasexcellent electrical and physical properties. They include,

(1) a high dielectric constant (ε) of not less than 8000

(2) a low dielectric loss (tan δ) of not more than 2.5%

(3) a high specific resistance of not less than 10¹¹ Ω-cm at roomtemperature.

(4) a low sintering temperature of 900° to 1000° C.

(5) The specific resistances at room temperature and an elevatedtemperature (85° C.) may be further increased by incorporation of aspecific amount of Mn. In this case, it is possible to obtain thespecific resistance at room temperature of not less than 10¹³ Ωcm.

Accordingly, the dielectric ceramic composition of the present inventioncan be used as a dielectric material not only for fixed ceramiccapacitors but also for monolithic ceramic capacitors. The dielectricceramic composition of the present invention has a low sinteringtemperature, thus making it possible to use a relatively inexpensivesilver-paradium alloy as a material for internal electrodes ofmonolithic ceramic capacitors. Accordingly, it is possible tomanufacture monolithic ceramic capacitors with a largecapacitance-to-volume ratio at a low cost.

What is claimed is:
 1. A dielectric ceramic composition consistingessentially of Pb(Ni_(1/3) Nb_(2/3))O₃, PbTiO₃, Pb(Zn_(1/2) W_(1/2))O₃and Pb(Cu_(1/3) Nb_(2/3))O₃, the mole percentages of said fourcomponents being as follows:Pb(Ni_(1/3) Nb_(2/3))O₃ --50.0 to 75.0 mole% PbTiO₃ --20.0 to 35.0 mole % Pb(Zn_(1/2) W_(1/2))O₃ --0.5 to 15.0 mole% Pb(Cu_(1/3) Nb_(22/3))O₃ --1.0 to 10.0 mole %
 2. The dielectricceramic composition according to claim 1 further containing, as anadditive, manganese in an amount of not more than 0.5 mole % in terms ofMnO₂ with respect to one mole of the basic composition consisting of50.0 to 75.0 mole % of Pb(Ni_(1/3) Nb_(2/3))O₃, 20.0 to 35.0 mole % ofPbTiO₃, 0.5 to 15.0 mole % of Pb(Zn_(1/2) W_(1/2))O₃ and 1.0 to 10.0mole % of Pb(Cu1/3Nb_(2/3))O₃.
 3. The dielectric ceramic compositionaccording to claim 1 wherein the composition has a high dielectricconstant of not less than 8000 and a low sintering temperature rangingfrom 900° to 1000° C.